In microscopy, image resolution is based, in part, on the wavelength of the energy used to interrogate an object. Conventional optical microscopy uses visible light to form a magnified image of a specimen. As a result, the image resolution that can be obtained is fundamentally limited by the wavelength of visible light, which includes wavelengths from about 400 nanometers (nm) to about 700 nm. In electron microscopy, on the other hand, a specimen is interrogated with a beam of electrons. Electrons are characterized by a wavelength (i.e., the de Broglie wavelength) that is many orders of magnitude smaller than that of visible light; therefore, electron microscopy enables significantly improved image resolution as compared to optical microscopy. In fact, the ability to image fine detail has led to electron microscopy becoming a mainstay in many applications, such as sub-cellular structural analysis in biological specimens, determination of the crystal structure, structural analysis of thin films, etc., where the resolution of optical microscopy is insufficient.
Electron microscopy encompasses a number of techniques based on different material-electron interactions that give rise to the transmission, reflection, absorption, emission, interference and/or diffraction of the electrons as they interrogate the material. Images generated using electron microscopy can be a “traditional image” (analogous to a visible-light photograph) or a “non-traditional image,” such as spectroscopic data that provides compositional information about the material. Perhaps the most ubiquitous electron microscopy technique capable of providing atomic resolution, however, is transmission Electron Microscopy (TEM). In TEM, electrons are transmitted through a thinned portion of a sample, referred to as a specimen. The specimen is typically disposed on a support membrane, although a specimen can be self-supported in some cases. As they pass through the specimen and support membrane, some of the electrons scatter and/or experience interference, giving rise to a “shadow image” of the specimen (and support membrane) in which sample structure manifests as varied contrast according to its density, thickness, or induced phase shift.
In order to obtain an image (traditional or non-traditional) of suitable quality, the material being imaged must be very thin and, ideally, of uniform thickness. One of the fundamental challenges in electron microscopy is the preparation required to form the thin specimen. Typically, specimens are prepared by hand or milled/ablated using a focused-ion beam (FIB) in conjunction with a precision mechanical stage and scanning electron microscope. Softer materials, such as biological samples, are typically sectioned using a glass or diamond edge to obtain a thickness within the range of a few nanometers to a few tens of microns. Specimens of harder materials, such as metals or semiconductor layers, are normally formed by thinning down a thick sample via the use of a physical and/or chemical subtractive process, such as etching, mechanical grinding, polishing, dimpling, ion milling, focused-ion-beam ablation, and the like. Unfortunately, conventional specimen preparation methods have several drawbacks.
First, conventional thinning processes are “blind” processes and, as a result, require frequent optical inspection to ensure the desired thinness is achieved and not exceeded. The removal of too much material often results in the specimen being damaged or destroyed. Prior-art specimen preparation methods provide no inherent protection against over-thinning of a sample. Further, not removing enough material can lead to substantial interference signals, during electron beam analysis, which can arise when the electron-beam-interaction volume contains a substantial volume-fraction of material other than the material of interest. Such interference signals can result, for example, from materials above (encapsulating material), or below (excess support structure), the material of interest, as well as laterally within the electron-beam-interaction volume.
Second, ideally, multiple prepared specimens of the same material-of-interest have the same thickness and are uniformly thin. Since specimens are generally prepared by hand and one at a time, however, variations in thickness and geometry within individual specimens are common and variation across different specimens can be significant. This can lead to undesirable imaging artifacts. Ion milling, for example, typically produces a wedge-shaped imaging region and thickness variations that can obscure features and produce artifacts in a resultant final image.
Third, the cost of specimen preparation in the prior art is extremely high due, in part, to the serial nature of conventional specimen preparation methods.
Fourth, many materials are not compatible with commercially available electron-microscopy grids that have pre-thinned support membranes. Vapor-phase-deposited materials, for instance, often deposit on all exposed surfaces—frontside and backside. This is particularly true for conformally deposited materials using deposition techniques such as atomic layer deposition (ALD), atomic layer epitaxy (ALE), other molecular layer deposition methods, vapor-phase epitaxy (VPE), metal-organic chemical vapor deposition (MOCVD), chemical-vapor deposition (CVD), low-pressure chemical-vapor deposition (LPCVD), plasma-enhanced chemical-vapor deposition (PECVD), etc. Although it is often possible to protect the back surface of a specimen membrane/grid during deposition using mechanical clamping, masking, etc., these solutions can be problematic and introduce issues with temperature stability, contamination, outgassing, induced stress, incompatibility with automated wafer handling and transport equipment, etc. In addition, the need for additional clamping/masking adds complexity and cost to the specimen preparation process.
A method for preparing large-area, very thin electron-microscopy specimens having precise and uniform thickness within a specimen, as well as across a plurality of specimens remains heretofore unrealized in the prior art.